The invention relates to an electrode composed of carbon having improved heat dissipation between electrode and workpiece.
Graphite electrodes are used in many different applications in industry. Examples thereof include aluminum and steel production, electrolysis of salt melts, electrolytic decomposition of chemical compounds, thermal deposition reactions, arc welding, measuring instruments and many more.
One important application in this case is the deposition of polysilicon according to the Siemens process, wherein high-purity elemental silicon is deposited from the gas phase on the surface of silicon rods. In this case, in a deposition reactor, from a mixture of hydrogen and halosilanes or a hydrogen-containing silicon compound, elemental silicon is deposited from the gas phase on the surface of a thin silicon rod heated to 900 to 1200° C.
In this case, the silicon rods are held in the reactor by specific electrodes, which generally consist of high-purity electrographite. In each case two thin rods having a different voltage polarity at the electrode mounts are connected at the other thin rod end by means of a bridge to form a closed electric circuit. Electrical energy for heating the thin rods is fed via the electrodes and the electrode mounts thereof. In this case, the diameter of the thin rods increases. At the same time, the electrode grows, starting at its tip, into the rod foot of the silicon rods. After a desired setpoint diameter of the silicon rods has been attained, the deposition process is ended, and the glowing silicon rods are cooled and demounted.
A particular importance is accorded here to the material and the shape of the electrodes. They serve, firstly, for retaining the thin rods, for transferring the current flow into the silicon rod, and also for transferring heat and also as a secure stage for the growing rod in the reactor. Since the trend is toward ever longer and heavier rods and the rod pairs, which in the meantime can have a weight of hundreds of kilograms, are only anchored by means of the electrodes in the reactor, precisely the choice of the shape and of the material constitution is very important.
Depending also on the subsequent use of the silicon rods thus produced, very different requirements are made of the silicon rods and the deposition process, and thus of the electrodes. If, by way of example, the polycrystalline silicon is subsequently used in silicon fragments for solar and electronics applications, the silicon rods must not fall over during or after the deposition process in the deposition reactor. Long and thick polycrystalline silicon rods increase the economic viability of the deposition process, but also the risk of falling over in the reactor.
Electrodes according to the prior art consist of a cylindrical base body in the lower part and a conical tip in the upper part. A cavity for receiving and making contact with the thin rod is provided at the conical tip. In this case, the lower end of the electrode is placed into a metallic electrode mount, via which the current is fed in. Such electrodes are generally known and are used for silicon deposition for example in U.S. Pat. No. 5,284,640.
Graphite is principally used as material for the electrodes since graphite can be produced with very high purity and is chemically inert under deposition conditions. Furthermore, graphite has a very low electrical resistivity.
U.S. Pat. No. 6,639,192 describes a graphite electrode having a conventional form. It consists of a cylindrical base body with a conical tip. The tip contains a hole for receiving and making contact with the thin rod. The electrode is produced from one part and thus from a material (here electrographite) having homogeneous material properties. It has a very high specific thermal conductivity, in particular. What is disadvantageous about this embodiment is a high rate of falling over before and during the deposition until the end diameter is attained.
DE 2328303 describes a cylindrical electrode without a tip. In this case, the carrier rod is inserted in a hole on a planar surface. This electrode form has a very high heat dissipation even in the case of a thin rod diameter on account of the fully cylindrical fall. In order that the rods with a thin diameter do not topple down during the deposition process, the electrode has to have a low heat dissipation, that is to say be of small diameter, and the electrode material has to have a very low specific thermal conductivity. Thick rods such as are usual nowadays cannot be deposited with this electrode form, since, on account of the small electrode diameter and the low specific thermal conductivity of the electrode material, the high energy necessary for thick rod diameters cannot be dissipated from the rod feet.
Graphite electrodes composed of a plurality of layers are known from other areas. However, in these cases, the arrangement of different layers is aimed at optimizing chemical conversions. U.S. Pat. No. 3,676,324 discloses for example a cylindrical graphite electrode consisting of a cylindrical inner and outer part, wherein the inner part has a very high electrical conductivity and the outer part is a porous graphite. The aim of this plurality of layers is to avoid high voltage losses and to obtain a high chemical conversion at the porous surface. A similar electrode having two different layers is known from GB 2135334, wherein here the outer porous layer serves for electrolytically obtaining fluorine.
What is disadvantageous about all the electrodes known from the prior art is that said electrodes, at the transition between electrode and the silicon rod or in the silicon rod in the vicinity of the electrode, tend to a greater or lesser extent to cracking or to chipping-off of material and thus make the silicon rod unstable.
Batches that have fallen over signify great economic damage. Thus, by way of example, in the event of the silicon rods falling over, damage to the reactor wall can occur. In this case, the silicon rods that have fallen over are contaminated by contact with the reactor and have to be cleaned on the surface. In addition, batches that have fallen over can only be demounted from the reactor with increased outlay. In this case, the surface of the silicon is contaminated further.
It was an object of the invention to provide an electrode with which the rate of falling over is significantly reduced by comparison with electrodes of conventional design.
It has surprisingly been found that an electrode composed of a plurality of different zones, consisting of materials having different thermal conductivities, does not have the disadvantages known from the prior art.